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Abstract:

Concentrated dispersion from 5 to 50% by weight of nanoparticles of
graphene in water with a lateral size from 10 to 5000 nm and thickness
from 0.34 to 30 nm. The production process comprises the dispersion in
water of flakes of expanded graphite and the subsequent treatment with
ultrasounds at an energy level of from 100 to 2000 W for a period from 1
to 100 hours.

Claims:

1. A dispersion of nanoparticles of graphene in water in the presence of
a surfactant: wherein: the concentration of said nanoparticles of
graphene in water is from 5% to 50% by weight; the C/O ratio in said
nanoparticles of graphene is ≧100:1; at least 90% of said
nanoparticles of graphene have a lateral size (x, y) from 10 to 10,000
nm, and a thickness (z) from 0.34 to 30 nm, the lateral size being always
greater than the thickness (x, y>z); and said surfactant is present in
an amount from 1 to 20% by weight with respect to the weight of said
nanoparticles of graphene and is an anionic surfactant, a nonionic
surfactant or combination thereof.

2. The dispersion according to claim 1, wherein the concentration of said
nanoparticles of graphene is from 7% to 50% by weight.

3. The dispersion according to claim 1, wherein said nanoparticles of
graphene have a lateral size (x, y) from 50 to 5,000 nm.

4. The dispersion according to claim 1, wherein said nanoparticles of
graphene have a thickness (z) from 0.34 to 20 nm.

5. The dispersion according to claim 1, wherein said C/O ratio in said
nanoparticles of graphene is greater than or equal to 150:1.

6. The dispersion according to claim 1, wherein said surfactant is
present in an amount from 5 to 15% by weight with respect to the weight
of said graphene.

7. The dispersion according to claim 1, wherein said surfactant is an
anionic surfactant.

8. A process for producing a dispersion of nanoparticles of graphene in
water, comprising expanding flakes of intercalated graphite having a
lateral size of less than or equal to 500 μm by exposure to a
temperature of at least 1,300.degree. C. for a time of less than 1
second, wherein the method further comprises: a) dispersing the expanded
graphite so obtained in water at a concentration from 5% to 50% by weight
to form a water dispersion in the presence of a surfactant in an amount
from 1 to 20% by weight with respect to the weight of said graphite, said
surfactant being an anionic surfactant, Of a nonionic surfactant or a
combination thereof: and b) treating the water dispersion obtained from
step a) with ultrasound at an energy level from 100 to 2000 W for a
period of from 1 to 100 hours.

9. The process according to claim 8, wherein said anionic surfactant
comprises a hydrophilic polar group which is an anion selected from the
group consisting of sulfonate and sulfate, and a hydrophobic nonpolar
part selected from the group consisting of benzene, naphthalene and
pyrene.

10. The process according to claim 8, wherein said anionic surfactant
comprises an anion of cholic acid selected from the group consisting of
cholate, deoxycholate and taurodeoxycholate.

11. The process according to claim 8, herein said nonionic surfactant is
selected from the group consisting of polyoxyethylene glycol alkyl
ethers, polyoxypropylene glycol alkyl ethers and polyvinylpyrrolidone.

12. The process according to claim 8, wherein said treating with
ultrasound of said step b) is performed at an energy level of from 200 to
1000 W for a period from 2 to 80 hours.

13. (canceled)

14. (canceled)

15. The dispersion according to claim 1, wherein the concentration of
said nanoparticles of graphene is from 7% to 40% by weight.

16. The dispersion according to claim 1, wherein said nanoparticles of
graphene have a lateral size (x, y) from 100 to 2,000 nm.

17. The dispersion according to claim 1, wherein said nanoparticles of
graphene have a thickness (z) from 0.34 to 15 nm.

18. A method comprising treating an article of manufacture with the
dispersion of claim 1.

19. An elastomeric composition comprising the dispersion of claim 1.

20. A paint comprising the dispersion of claim 1.

21. A textile article comprising the dispersion of claim 1.

Description:

[0001] The present invention relates to a concentrated water dispersion of
graphene and to a method for the preparation thereof.

[0002] Graphene is a material formed by a single atomic layer of sp2
hybridized carbon atoms. These are arranged in hexagonal close-packed
honeycomb structures that form the fundamental structural elements of
graphite, of carbon nanotubes and of fullerenes.

[0003] Graphene is a material with unique properties: it is a zero
band-gap semiconductor with high charge carrier mobility (up to 200,000
cm2/Vs), very high mechanical strength (tensile strength ˜40
N/m, Young's Modulus ˜1.0 TPa), exception thermal conductivity
(˜5000 W/Km) and high electric current carrying capacity
(˜1.2 mA/μm). These properties allow graphene to be used for
applications in market segments that require the use of advanced
materials. Therefore, graphene based materials are studied from a
scientific and industrial point of view for applications in markets such
as electronics, photovoltaics, batteries, sensors, optoelectronics and
nanocomposites.

[0004] Scientific and patent literature describes various methods for the
preparation of graphene, such as chemical vapor deposition, epitaxial
growth, chemical exfoliation and chemical reduction of the oxidized form
graphene oxide (GO).

[0005] The Applicant Directa Plus S.p.A. is the holder of European patent
EP 2 038 209 B1, which describes and claims, among other things, a method
for producing structures comprising graphene layers, obtained by
intercalation and subsequent expansion/exfoliation of graphite.

[0006] US 2011/0017585 A1 describes the production of nano graphene
platelets (NGPs,) by means of ultrasonication of pristine graphite
dispersed in a liquid without surfactants. The liquid used must have a
low surface tension, to ensure high wettability of the graphene. By
studying the surface tension of many solvents it was found that the
thickness of the nano graphene platelets obtained depended on the contact
angle with the liquid, defined "solvent". All solvents used were organic
solvents. The solvents used in the examples were n-heptane, glycerol and
benzene. The description indicates the possibility of obtaining
dispersions having a concentration of starting material (graphite) higher
than 0.1 mg/mL, generally higher than 1 mg/mL, more frequently higher
than 10 mg/mL (1% weight). The examples refer to dispersions having
concentrations of 0.5% weight.

[0007] US 2008/0279756 A1 describes a method of producing exfoliated
graphite, flexible graphite and nano graphene platelets (NGPs). The
method comprises the dispersion of graphite particles or of graphite
oxide in water followed by ultrasonication treatment at an energy level
sufficient to generate platelets of nanometric dimension. The description
(paragraph

[0042]) refers to particles with lateral sizes lower than 100
nm. Example 5 describes ultrasonication treatment of a water dispersion
of NGPs at 2% by weight, in the presence of a surfactant, but does not
indicate the dimensions of the NGPs obtained. Dispersions with graphene
concentration higher than 2% are not described.

[0008] U.S. Pat. No. 8,222,190 B2 describes a nano-graphene modified
lubricant, comprising nano graphene platelets (NGPs) dispersed in a
lubricating fluid based on petroleum oil or synthetic oil at a
concentration from 0.001% to 75% by weight, preferably from 0.001% to 60%
by weight.

[0009] Example 1 describes the production of ultra-thin graphene sheets by
means of ultrasonication of graphite flakes dispersed in water at the
concentration of 0.5% for 2 hours and subsequent spray-drying of the
dispersion obtaining dried NPGs with a thickness of 1-5 graphene layers.

[0010] Example 4 describes the production of various lubricant
compositions comprising NPGs dispersed in α-olefin or polyolefin at
the concentration of 2.5% by weight (sample 1) and 45% by weight (sample
5).

[0011] Therefore, dispersions of graphene either at low concentration or
comprising organic solvents or even liquids formed by α-olefin
oligomers are known. In fact, the hydrophobic nature of graphene means
that the use of water is avoided as liquid medium of the dispersion. This
hydrophobic nature instead leads to the use of organic solvents, which
are both costly and problematic from a safety and environmental point of
view.

[0012] However, the majority of industrial applications of graphene
mentioned above rely on the availability of graphene preferably in a form
that is concentrated, easy-to-use, relatively inexpensive and safe from a
health and environmental point of view.

[0013] An object of the present invention is therefore to provide a
dispersion of graphene in relatively concentrated form, produced with a
liquid medium that is readily available, inexpensive and very safe from a
health and environmental perspective.

[0014] Another object of the present invention is to provide a safe and
reliable process for preparing a dispersion of graphene as described
above that enables graphene to be obtained in a very pure form and with a
minimum amount of oxidized product.

[0015] The aforesaid and other objects and advantages of the invention are
achieved with a dispersion of nanoparticles of graphene in water in the
presence of a surfactant, characterized in that:

[0016] the
concentration of the nanoparticles of graphene in water is from 5% to 50%
by weight;

[0017] the C/O ratio in said nanoparticles of graphene is
≧100:1;

[0018] at least 90% of said nanoparticles of graphene have
a lateral size (x, y) from 10 to 10,000 nm, and a thickness (z) from 0.34
to 30 nm, the lateral size being always greater than the thickness (x,
y>z);

[0019] the surfactant is present in an amount from 1 to 20% by
weight with respect to the weight of said graphene and is an anionic
surfactant or a nonionic surfactant or a mixture of these.

[0020] According to the invention, a process for producing a dispersion of
nanoparticles of graphene in water comprises the expansion of flakes of
intercalated graphite having a lateral size of ≦500 μm by
exposure thereof to a temperature of at least 1300° C. for a time
of less than 1 second, and is characterized in that:

[0021] a) the
expanded graphite so obtained is dispersed in water at a concentration
from 5% to 50% by weight, in the presence of a surfactant in an amount
from 1 to 20% by weight with respect to the weight of said graphite, said
surfactant being an anionic surfactant or a nonionic surfactant or a
mixture of these;

[0022] b) the water dispersion obtained from step a) is
treated with ultrasounds at an energy level from 100 to 2000 W for a
period of from 1 to 100 hours.

[0023] The aforesaid process enables a dispersion of nanoparticles of
graphene having the characteristics defined above to be obtained.

[0024] In the present description the size of the nanoparticles of
graphene is defined with reference to a system of Cartesian axes x, y, z,
it being understood that the particles are substantially flat platelets
but may also have an irregular shape. In any case, the lateral size and
the thickness provided with reference to the directions x, y and z must
be intended as the maximum sizes in each of the aforesaid directions.

[0025] The lateral sizes (x, y) of the nanoparticles of graphene are
determined by direct measurement on the scanning electron microscope
(SEM), after having diluted the dispersion in a ratio of 1:1000 in
deionized water and added it dropwise to a silicon oxide substrate placed
on a plate heated to 100° C.

[0026] The thickness (z) of the nanoparticles of graphene is determined
with the atomic force microscope (AFM), which is essentially a
profilometer with subnanometer resolution, widely used for
characterization (mainly morphological) of surfaces and of nanomaterials.
This type of analysis is commonly used (both for academic purposes and in
industrial R&D) to evaluate the thickness of the graphene flakes,
produced with any method, and to detect the number of layers forming the
flake (single layer=0.34 nm).

[0027] The particles of the dispersion, deposited as described for SEM
analysis, are scanned directly with an AFM tip, where the measurement
provides a topographical image of the graphene flakes and their profile
with respect to the substrate, enabling precise measurement of the
thickness.

[0028] In the dispersion according to the invention at least 90% of the
nanoparticles of graphene preferably have a lateral size (x, y) from 50
to 5,000 nm, more preferably from 100 to 2,000 nm.

[0029] In the dispersion according to the invention the nanoparticles of
graphene preferably have a thickness (z) from 0.34 to 20 nm, more
preferably from 0.34 to 15 nm.

[0030] In any case, the lateral size is always greater than the thickness
(x, y>z).

[0031] In the dispersion according to the invention the C/O ratio in said
nanoparticles of graphene is ≧100:1; preferably ≧150:1.
This ratio is important as it defines the maximum amount of oxygen bonded
to the carbon forming the graphene, i.e. of graphene oxide. It is in fact
known that the best properties of graphene are obtained when the amount
of graphene oxide is minimum. On the other hand, the polar character of
the graphene oxide makes it more hydrophilic and therefore suitable for
the formation of water dispersions. One of the technical problems solved
by the invention is therefore that of obtaining concentrated dispersions
of graphene in water while maintaining the content of graphene oxide very
low, as defined above.

[0032] The C/O ratio in the graphene of the dispersion according to the
invention is determined by elementary analysis performed by Inductively
Coupled Plasma Mass Spectrometry (ICP-MS), which provides the percentage
by weight of the various elements. By normalizing the values obtained
with respect to the atomic weight of the C and O species and finding
their ratio, the C/O ratio is obtained.

[0033] The term "concentrated dispersion of graphene in water" is intended
as a dispersion from 5% to 50% by weight of nanoparticles of graphene,
preferably from 7% to 50% by weight, more preferably from 7% to 40% by
weight.

[0034] Dispersions having these concentrations of graphene are
advantageous for the use of graphene in numerous industrial applications,
such as:

[0035] use as additive or component of elastomeric
compositions for tires, where it is advantageous to reach graphene levels
of 10-15% by weight in order to achieve desired properties such as: i) an
improved gas barrier effect, with consequent increase of impermeability,
causing the tire to deflate more slowly; ii) improvement of the
mechanical dynamic properties, in particular rolling resistance; iii)
increase of thermal conductivity, useful for heat dissipation; iv)
increase of electrical conductivity, useful for the dissipation of
electrostatic energy;

[0036] use as additive or component of paints and
silicon compositions, where it is advantageous to reach graphene levels
of 20-30% by weight in order to achieve desired properties, such as: i)
increase of thermal conductivity for heat dissipation, ii) increase of
electrical conductivity, to reach conductive compounds; iii) gas and
liquid barrier effect with consequent increase of impermeability, to give
anticorrosion and antifouling properties;

[0037] use as additive or
component in compositions for treating articles such as textiles, where
it is advantageous to reach graphene levels of up to 40% by weight in
order to achieve desired properties, such as: i) good electrical
conductivity, for producing "intelligent" textiles; ii) good thermal
conductivity; iii) liquid barrier effect; iv) flame retardant properties;
v) EM and IR shielding.

[0038] In some applications, for example in the treatment of textiles, the
direct use of suspensions of graphene in water is possible, as the
preparation of a dispersion or suspension to apply to the textile is in
any case required, according to various technical processes available to
those skilled in the art.

[0039] In the dispersion according to the invention the surfactant is
present in an amount from 1 to 20% by weight with respect to the weight
of said graphene, preferably from 5 to 15% by weight. It has the function
of dispersing medium and contributes to the stability of the dispersion
in time. The surfactant is selected from anionic surfactants or nonionic
surfactants or mixtures thereof. Preferred anionic surfactant are those
in which the anion forming the hydrophilic polar group is selected from
sulfonate, sulfate, carboxylate and the hydrophobic nonpolar part is
selected from structures comprising aromatic rings such as benzene,
naphthalene, pyrene or cyclic aliphatic structures such as derivatives of
cholic acid. Suitable derivatives of cholic acid are the anions
deoxychlate (DOC) and taurodeoxycholate (TDOC). A particularly preferred
surfactant is sodium benzenesulfonate.

[0041] The dispersion of graphene according to the invention is prepared
with a process involving several steps.

[0042] The first step of the process consists in the preparation of
expanded and/or exfoliated graphite starting from intercalated graphite.

[0043] The intercalated graphite can be prepared with methods known to
those skilled in the art or purchased on the market. The expansion step
of the intercalated graphite is performed by exposing flakes of
intercalated graphite (Graphite Intercalation Compounds, GICs) having a
lateral size ≦500 μm to a temperature of at least 1300°
C. for a time of less than 1 second. This treatment is performed as
described in EP 2 038 209 B1, i.e. by generating heat within the GICs,
preferably using an electric arc, a microwave or induction furnace at
high frequency or by plasma formation. This last treatment is
particularly preferred as it is possible to reach the temperature
required associated with high turbulence.

[0044] The second step of the process comprises the dispersion in water of
the expanded graphite obtained in the first step. Dispersion is obtained
by light stirring. Dispersion is performed in the presence of a
surfactant in an amount from 1 to 20% by weight with respect to the
weight of the graphite, preferably from 5 to 15% by weight. The
surfactant is an anionic or nonionic surfactant, preferably an anionic
surfactant in which the anion forming the hydrophilic polar group is
selected from sulfonate, sulfate, carboxylate and the hydrophobic
nonpolar part is selected from structures comprising aromatic rings such
as benzene, naphthalene, pyrene or cyclic aliphatic structures such as
derivatives of cholic acid. A particularly preferred surfactant is sodium
benzenesulfonate.

[0045] The expanded graphite is dispersed in water at a concentration from
5% to 50% by weight, preferably from 7% to 50% by weight, more preferably
from 7% to 40% by weight.

[0046] The third step of the process comprises treatment of the water
dispersion obtained in the preceding step with ultrasounds at an energy
level from 100 to 2000 W for a period of from 1 to 100 hours.

[0047] Preferably the treatment of the water dispersion of expanded
graphite with ultrasounds is performed an energy level from 200 to 1000 W
for a period of from 2 to 80 hours.

[0048] The treatment with ultrasounds is performed using an apparatus such
as commercial ultrasonicators for treating liquids, where the acoustic
energy is transmitted to the system by cavitation (formation and
implosion of bubbles) using a sonotrode immersed in the liquid, with wave
frequency of around 24 kHz, and with power as defined above.

[0049] The combination of the expansion treatment of the intercalated
graphite at high temperature and of the subsequent ultrasonication
treatment in a water medium enables both an exfoliation of the graphite
and a reduction in the size thereof to be performed, obtaining
nanoparticles of graphene directly dispersed in water, in relatively
rapid times.

[0050] Moreover, the process described above makes it possible to obtain
dispersions of graphene in water having higher concentrations than those
obtained with prior art processes.

[0051] The availability of concentrated dispersions having up to 50%
weight of nanoparticles of graphene of very small size represents a
substantial improvement compared to the prior art, both from the point of
view of performance of the dispersion and of its processability.

[0052] From the point of view of performance, it was found that the fine
particles of graphene of very small size interact optimally with the host
matrix to which the dispersion is applied, whatever it be. The relatively
limited amount of surfactant interferes negligibly with the physical
properties of the graphene.

[0053] From the point of view of processability it was found that the high
concentration makes it possible to reduce the volumes of suspension
treated with the same amounts of graphene to be applied to a given
substrate, making the process of applying the graphene to the substrate
or matrix involved more manageable and less costly.

[0054] The invention will now be described by means of some embodiments
provided merely by way of example.

EXAMPLE 1

[0055] 75 g of grade ES 250 F5 commercially available intercalated
graphite (hereinafter IG), marketed by Graphit Kropfmuhl AG, having a
lateral size of around 300 μm, were expanded by insertion into an
induction plasma with the following characteristics:

[0056] The expansion temperature of was 1300° C. and the transit
time was around 0.2 seconds. The resulting expanded graphite (EG) had an
apparent density of ˜2.5 g/and a C/O ratio of around 150:1. The
expanded graphite was then dispersed in 1000 mL of deionized water
containing naphthalene sulfonate as dispersing agent in the amount of 10%
by weight with respect to the amount of expanded graphite, until
obtaining a suspension. The dispersing agent was composed of an aromatic
nonpolar group (naphthalene), having high affinity for graphite, and a
polar group (sulfonate) that promotes graphite/water affinity.

[0057] For the treatment with ultrasounds, which produces exfoliation and
reduction in size of the expanded graphite, an energy level of 400 W
(UIP400S, Hielscher) for a duration of 15 hours was used.

[0058] The final dispersion had a graphene concentration of 7.5% by
weight. This dispersion was diluted 1:1000 in deionized water and added
dropwise to a silicon oxide substrate placed on a plate heated to
100° C. The substrate was analyzed with the scanning electron
microscope (SEM). It was found that the nanographene platelets had a
lateral size in the range of 500-3000 nm, and a thickness in the range of
0.34-15 nm.

EXAMPLE 2

[0059] The procedure of example 1 was repeated with the following
variations.

[0060] 100 g of the same commercial IG were expanded at an ultrasound
energy level of 400 W (UIP400S, Hielscher) obtaining exfoliation and size
reduction for a period of 30 hours. The final dispersion had a graphene
concentration of 10% by weight.

[0061] The final dispersion was diluted 1:1000 in deionized water and
added dropwise to a silicon oxide substrate placed on a plate heated to
100° C. The substrate was analyzed with the scanning electron
microscope (SEM). It was found that the nanographene platelets had a
lateral size in the range of 200-2000 nm, and a thickness in the range of
0.34-10 nm.

EXAMPLE 3

[0062] The procedure of example 1 was repeated with the following
variations.

[0063] 200 g of the same commercial IG were expanded at an ultrasound
energy level of 400 W (UIP400S, Hielscher) obtaining exfoliation and size
reduction for a period of 60 hours. The final dispersion had a graphene
concentration of 20% by weight.

[0064] The final dispersion was diluted 1:1000 in deionized water and
added dropwise to a silicon oxide substrate placed on a plate heated to
100° C. The substrate was analyzed with the scanning electron
microscope (SEM). It was found that the nanographene platelets had a
lateral size in the range of 100-1000 nm, and a thickness in the range of
0.34-6 nm.

EXAMPLE 4

[0065] 200 g of commercial intercalated graphite flakes (IG), purchased
from GK (ES 250 F5 grade), having a lateral size of approximately 300
μm, were expanded by feeding the IG through an Argon gas plasma
(feeding rate 5 g/min, Ar flow: Plasma=15 l/min, Auxiliary=1.5 l/min,
Carrier=1 l/min, RF=40 MHz, P˜1400 W). The plasma was generated
with an ICP-OES from Agilent (710 Series), where the spectrometer stage
was removed. The expansion temperature was at least 1300° C. and
the transit time of approximately 0.2 second. The resulting expanded
graphite (EG) had an apparent density of ˜2.5 g/l and a C/O ratio
of about 20:1. The EG was then dispersed in 1,000 mL of deionized water,
containing 10% by EG weight of the anionic surfactant sodium cholate, to
obtain a suspension. The nonionic surfactant comprises an apolar aromatic
group, with high affinity to graphite lattice, and a polar group which
promotes graphite/water affinity. An ultrasonic energy level of 400 W
(UIP400S, Hielscher) was used for exfoliation and particle sizes
reduction for a period of 60 hours.

[0066] Since there was a tendency to foaming, tributoxyethyl phosphate as
deaerating agent was added to the dispersion in equal amount (i.e. 10% by
EG weight) as the nonionic surfactant. Both sodium cholate and
tributoxyethyl phosphate were purchased from Sigma Aldrich.

[0067] The final dispersion was diluted 1:1,000 in deionized water and
drop-casted onto a silicon dioxide substrate, placed on top of an
hot-plate at 100° C. The substrate was analysed by scanning
electron microscopy (SEM), showing an average nano graphene sheets
lateral size in the range 500-10,000 nm, and a thickness in the range
0.5-15 nm.

EXAMPLE 5

[0068] The procedure of Example 4 was repeated with the following
parameters.

[0069] 200 g of the same commercial IG was expanded The EG was then
dispersed in 1,000 mL of deionized water, containing 10% by EG weight of
the non ionic surfactant polyvinylpyrrolidone as dispersing agent, to
obtain a suspension. An ultrasonic energy level of 400 W (UIP400S,
Hielscher) was used for exfoliation and particle sizes reduction for a
period of 60 hours. The final dispersion was diluted 1:1000 in deionized
water and drop-casted onto a silicon dioxide substrate, placed on top of
an hot-plate at 100° C. The substrate was analysed by scanning
electron microscopy (SEM), showing an average nano graphene sheets
lateral size in the range 500-5,000 nm and a thickness in the range
0.5-10 nm.

Patent applications in class Coating or impregnation is specified as water proof

Patent applications in all subclasses Coating or impregnation is specified as water proof